Digital oscilloscopes (and other digital signal acquisition instruments) sample analog input signal voltages, convert each analog sample to a digital value with an analog-to-digital converter, store the digital sample values in a memory for processing, and display the processed digital sample values. A waveform record is a one dimensional array of digital sample values. Typically, the array is a time ordered list, with a fixed time increment between adjacent samples. Modern digital oscilloscopes typically have three sampling methods for acquiring a waveform and filling in the waveform record: single-shot sampling, sequential repetitive sampling, and random repetitive sampling.
In single-shot sampling (also called real time sampling), an entire waveform record is captured from one trigger event. Enough samples occur within the time interval of interest to adequately represent the analog signal that was sampled. The time between adjacent samples is the reciprocal of the sample rate.
In sequential repetitive sampling, one sample is typically captured from each trigger event. The time from the trigger event to the sample is typically incremented by a small amount for each subsequent trigger event. The waveform record is filled sequentially in time order, requiring one trigger for each record entry. The time between adjacent samples in the waveform record is much shorter than the reciprocal of the sample rate.
In random repetitive sampling, multiple samples are captured from each trigger event, but not enough to fill the waveform record. The samples are taken at random times with respect to the trigger event. The time from the trigger event to each sample is accurately measured and each sample is placed at the proper time slot in the waveform record. With multiple trigger events, the waveform record is gradually filled. The time between adjacent samples in the waveform record is much shorter than the reciprocal of the sample rate.
When a waveform record is drawn or displayed, individual samples may be represented simply as dots or pixels. Alternatively, it is often useful to connect dots. One straightforward algorithm for connecting dots is to simply connect the dot for sample (N) to the dot for sample (N+1). For single-phase waveforms, for example a sine wave or a square wave, simple sequential connection of dots enhances the display. However, some waveforms are multi-phase. That is, they may have multiple values at a given delay time from a trigger event. One common example is the output of a digital gate that at a given delay from a clock edge may have a voltage corresponding to a logical one or a voltage corresponding to a logical zero. For sequential repetitive sampling or random repetitive sampling, sample (N) may come from a different trigger event than sample (N+1). Therefore, for example, sample (N) may be from a logical one state and sample (N+1) may be from a logical zero state. A human observer viewing just dots or pixels is capable of appropriately interpreting the two separate states. However, simply connecting adjacent record values, where adjacent record values randomly represent one of two states, can result in a chaotic display. There is a need for an improved method of connecting samples of multi-phase signals when drawing or displaying samples from sequential or random repetitive samples.